We support "./foobar.deb" as a way to install a deb file directly.
Recently .changes files were added. This highlights a problem as you
can't add the changes file without also trying to install all of them.
Now, it could also be handy to add entire Packages/Sources files to
perhaps get a bunch of packages in without installing them all
This commit introduces --with-source which allows to add *.deb, *.changes,
*.dsc, source-dirs, Packages & Sources files (the later can also be
compressed) without also installing them.